Journal of Alloys and Compounds 431 (2007) L4–L7
Letter
Facile preparation, characterization and optical properties
of rectangular PbCrO4 single-crystal nanorods
Bei Cheng, Hua Guo, Jiaguo Yu ∗ , Xiujian Zhao
State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology,
Luoshi Road 122#, Wuhan 430070, PR China
Received 2 May 2006; received in revised form 11 May 2006; accepted 13 May 2006
Available online 3 July 2006
Abstract
Monoclinic PbCrO4 nanorods, with a rectangular cross section, a typical length of 6–7 ␮m, a width of 80–150 nm, and a width-to-thickness ratio
of 2–5, have been successfully synthesized via a simple precipitation reaction, followed by a hydrothermal treatment without using any templates and
additives. The as-synthesized products were characterized with X-ray diffraction, transmission electron microscopy, high-resolution transmission
electron microscopy, scanning electron microscopy and selected area electron diffraction, UV–vis absorption spectra and photoluminescence
spectra. It was found that the length of as-obtained PbCrO4 nanorods relies on the pH value and the aging temperature, and the optical properties of
PbCrO4 nanorods depend on their length. The intensities of UV–vis absorbance of PbCrO4 nanorods slightly decrease with increasing the length
of nanorods. On the contrary, the intensities of room-temperature photoluminescence of PbCrO4 nanorods increase with increasing the length of
nanorods.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Lead chromate; Nanorod; Rectangle; Preparation; Hydrothermal reaction; Characterization; Optical property
1. Introduction
Lead chromate (PbCrO4 ) is an important photo-electricity
solid functional material that has been widely used in decorative systems, protective systems, and mass coloration of fibers,
plastics, papers, elastomers and rubbers [1–3]. In addition, it has
also been used as a host material for photosensitizer, humiditysensing resistor and so forth [4,5]. Usually, PbCrO4 exists in
two kinds of crystal structures: the stable monoclinic structure
and the unstable orthorhombic structure [1,6]. Since the discovery of carbon nanotubes in 1991 [7], one-dimensional (1D)
nanostructured materials (nanotubes, nanobelts, nanowires, and
nanorods) have attracted considerable attention from the scientific community due to their distinctive geometries, novel
physical and chemical properties, and potential applications in
numerous areas such as nanoscale electronics and photonics
[8–11]. Recently much effort has been directed toward under-
∗
Corresponding author.
E-mail addresses: [email protected] (J. Yu), [email protected]
(X. Zhao).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.05.096
standing the electronic, magnetic, and optical properties of these
nanostructures because these unique size- and shape-dependent
properties are physically or chemically different from their bulk
counterparts [12–14]. Therefore, the synthesis of lead chromate
(PbCrO4 ) nanorods with well-controlled size and shape is of
great significance for its applications.
To date, there have been a few reports on the preparation of
lead chromate nanorods [15–19], but few efforts were put to synthesize uniformly ultra-long and rectangular PbCrO4 nanorods,
and to investigate their related optical properties. Herein, we
develop a facile hydrothermal route for the synthesis of rectangular PbCrO4 nanorods without using any templates or additives.
Excitingly, the length of the as-obtained PbCrO4 nanorods can
be easily manipulated ranging from several hundred nanometers
to several micrometers by varying the aging temperature and pH
value. The PbCrO4 nanorods show the photoluminescence (PL)
emission and the intensity of photoluminescence increases with
increasing the lengths of PbCrO4 nanorods, while the emissionband shape remains the same. To the best of our knowledge,
this is the first report on the synthesis of rectangular PbCrO4
nanorods with controllable length and luminescence intensity
so far.
B. Cheng et al. / Journal of Alloys and Compounds 431 (2007) L4–L7
2. Experimental
All chemicals used were of analytical grade and used as received without
further purification. Aqueous solutions of K2 CrO4 (0.5 M) and of Pb(NO3 )2
(0.5 M) were first prepared as stock solutions. In a typical synthesis, 0.2 ml
K2 CrO4 aqueous solution (0.5 M) was added into 50 ml distilled water with a pH
value of 3. Then, 0.2 ml Pb(NO3 )2 aqueous solution (0.5 M) was quickly injected
into the above solution under continuous stirring by using magnetic stirrer. This
gave a final PbCrO4 concentration of 2 mM. The mixture was further stirred for
5 min and then transferred into a 50 ml Teflon-lined stainless steel autoclave.
The autoclave was heated and kept at 150 ◦ C for 24 h, and then was allowed to
cool to room temperature. Finally the obtained products were centrifuged and
washed with distilled water and absolute ethanol for several times. The final
samples were dried in a vacuum at 80 ◦ C for 6 h. As reference experiments,
the samples were also synthesized with variation of pH value and temperatures,
while all other conditions were kept the same.
The morphologies of resulting PbCrO4 products were characterized with
scanning electron microscopy (SEM, JSM-5610LV, Japan), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy
(HRTEM) (JEOL-2010F at 200 kV, Japan). The powder X-ray diffraction (XRD)
patterns were obtained on an HZG41B-PC diffractometer using Cu K␣ radiation
at a scan rate of 0.05◦ 2θ S−1 to characterize the crystalline phase of the products. UV-visible (UV–vis) absorption spectra of the as-prepared PbCrO4 solid
powders were recorded for the dry-pressed disk samples at room temperature
on a UV–vis spectrometer (UV-2550, Shimadzu, Japan). BaSO4 was used as an
absorption standard in the UV–vis absorption experiment. The photoluminescence (PL) spectra of the as-prepared PbCrO4 solid powders were measured at
room temperature on a Perkin-Elmer LS 50B luminescence spectrometer using
a 400 nm excitation line.
3. Results and discussion
The XRD pattern of PbCrO4 nanorods obtained in a typical
synthesis condition is shown in Fig. 1a. All the reflection peaks
can be readily indexed to a pure monoclinic phase (space group:
P21 /n (14)) of PbCrO4 with lattice constants a = 0.712 nm,
b = 0.743 nm, c = 0.679 nm and β = 102.42◦ , which are in good
agreement with the literature values (JCPDS: 73-2059). The
reaction between K2 CrO4 and Pb(NO3 )2 can be expressed as
follows:
K2 CrO4 + Pb(NO3 )2 → PbCrO4 ↓ + 2KNO3
(1)
Fig. 1. XRD patterns of PbCrO4 nanorods obtained at different pHs and aging
temperatures: (a) pH 3, T = 150 ◦ C; (b) pH 7, T = 150 ◦ C; (c) pH 7, T = 25 ◦ C.
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The morphology and size of the resulting product are observed
with SEM and TEM. Fig. 2a shows a representative SEM image
of the products obtained in a typical synthesis condition. It can
be seen that the products are predominantly composed of long
and straight nanorods, with a typical length of 6–7 ␮m, a size of
80–150 nm, and a high length-to-size ratio of 40–80. The structures of the products were further examined using transmission
electron microscopy (TEM) and high-resolution transmission
electron microscopy (HRTEM). Fig. 2b shows the low magnification TEM image of typical PbCrO4 nanorods. As can be
seen from Fig. 2b, the nanorods show uniform size over their
entire lengths, and some of the nanorods stick together. Fig. 2c
shows an amplified TEM image of the as-prepared PbCrO4
nanorod. It can be seen that cross-section of PbCrO4 nanorod
is rectangle. After carefully examining many PbCrO4 nanorod
samples, it was found that the width, thickness and a widthto-thickness ratio of PbCrO4 nanorods are about 80–150 nm,
30–50 nm and 2–5, respectively. The selected area electron
diffraction (SAED) pattern (inset in Fig. 2c) taken from a single nanorod and recorded from the [0 1 0] zone axis indicates
that the nanorods are single crystals with a preferential growth
direction along the [1 0 1] direction. Fig. 2d is a typical HRTEM
image of a single-crystalline PbCrO4 nanorod, which shows
the clearly resolved interplanar distance d−2,0,2 = 0.269 nm and
d3 0 1 = 0.203 nm, and further confirms that the nanorods grow
along the [1 0 1] direction.
To find out the influence of pH value and aging temperature on the morphology and size of PbCrO4 nanorods, a set
of experiments were carried out with variation of pH value
and temperatures. When pH value was decreased and other
reaction conditions were kept the same, no obvious morphological change was observed for PbCrO4 nanorods (corresponding
SEM pictures are not shown here). However, when pH value
was increased to 7, the as-obtained products were mainly short
rectangular nanorods with an average length of about 2–3 ␮m
(Fig. 3a). The corresponding XRD pattern of the as-obtained
nanorods at pH 7 is shown in Fig. 1b. Comparing Fig. 1b with a,
it can be found that the two XRD patterns are almost the same
except for the obvious weakening of the (1 2 0) diffraction peak
in Fig. 1b, indicating that the obtained products in this case are
still monoclinic PbCrO4 . This confirms that adjusting pH value
will not affect the phase composition of PbCrO4 crystal, and a
low pH value is favorable for the crystallization and growth of
PbCrO4 nanorods. This may be due to the fact that a low pH
value extends the induction time of PbCrO4 precipitation [20],
but how does pH exactly affect on the formation of PbCrO4
nanorods is still unclear.
Further investigation showed that when the aging temperature
was varied to room temperature (25 ◦ C) and all other conditions kept the same, only some shorter rectangular nanorods
with a length range from several hundred nanometers to several
micrometers were obtained (as shown in Fig. 3b). Corresponding XRD pattern (as shown in Fig. 1c) can also be well indexed to
monoclinic PbCrO4 phase. On the basis of the above experiment,
it can be concluded that a low aging temperature is unfavorable
for the growth of PbCrO4 nanorods, but has no influence on the
phase structure. This may be ascribed to the lack of Ostwald
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B. Cheng et al. / Journal of Alloys and Compounds 431 (2007) L4–L7
Fig. 2. SEM image (a), TEM image (b), amplified TEM image (c), SAED pattern (inset of (c)) and HRTEM image (d) of the resulting product prepared in a typical
synthesis condition.
ripening [21], in which relatively large stable nuclei grow at
the expense of the smaller unstable nuclei at high temperature.
Therefore, it is reasonable to believe that low pH value and high
aging temperature are crucial to the formation of uniformly long
PbCrO4 nanorods. Aging temperature and pH value can influence the nucleation and the growth of PbCrO4 crystal, which
determine the final morphology of the products.
The optical properties of as-synthesized PbCrO4 nanorods
were also studied. Fig. 4 shows the UV–vis absorption spectra
of the samples obtained at different pH values and aging temperatures. It can be seen that the shapes of the absorption spectra are
very similar. All the curves show two broad absorption bands in
the wavelength range from 250 to 600 nm, peaking at about 360
and 470 nm, respectively. Further observation (from Figs. 2–4)
shows that the intensities of absorbance slightly decrease with
increasing the length of nanorods. This may be due to the fact
that the smaller the size of particles, the larger the surface area
will be, which leads to a stronger absorbance.
The iso-electronic system CrO4 2− was ever accepted to
be non-luminescent because of rapid radiationless deactivation
[22], until the discovery of photoluminescence from CaCrO4 ,
and SrCrO4 crystals in 1980s [23,24]. The photoluminescence of
these chromates may be attributed to emission from a metastable
triplet state of the chromate ion [19]. In our experiment, the
photoluminescence properties of PbCrO4 nanorods were investigated at room temperature and the corresponding photoluminescence spectra are shown in Fig. 5. It can be seen that the three
samples have similar emission peaks at 485, 528 and 542 nm
(excitation wavelength: 400 nm). By the comparison of the three
curves in the spectra, it can be also seen that the photoluminescence intensities increase with increasing the length of PbCrO4
nanorods. Such a trend is reverse to that of the UV–vis spectra.
Fig. 3. SEM images of PbCrO4 nanorods obtained at different pHs and aging temperatures: (a) pH 7, T = 150 ◦ C; (b) pH 7, T = 25 ◦ C.
B. Cheng et al. / Journal of Alloys and Compounds 431 (2007) L4–L7
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thickness ratio of 2–5, using a simple hydrothermal method in
a typical synthesis condition. It has been demonstrated that the
morphological uniformity and length of the as-obtained PbCrO4
nanorods rely on the pH and the aging temperature. The UV–vis
absorbances of PbCrO4 nanorods slightly decrease with increasing the length of nanorods. On the contrary, the intensities
of room-temperature photoluminescence of PbCrO4 nanorods
increase with increasing the length of nanorods.
Acknowledgements
Fig. 4. UV–vis absorption spectra of the products obtained at different pHs
and aging temperatures: (a) pH 3, T = 150 ◦ C; (b) pH 7, T = 150 ◦ C; (c) pH 7,
T = 25 ◦ C.
This work was partially supported by the National Natural Science Foundation of China (20473059). This work was
also financially supported by the Key Research Project of Chinese Ministry of Education (No. 106114) and Program for
Changjiang Scholars and Innovative Research Team in University (PCSIRT), Ministry of Education, China.
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Fig. 5. Room-temperature photoluminescence spectra of PbCrO4 nanorods
obtained at different pHs and aging temperatures and recorded at an excitation wavelength of 400 nm: (a) pH 3, T = 150 ◦ C; (b) pH 7, T = 150 ◦ C; (c) pH
7, T = 25 ◦ C.
The result further confirms that the length of PbCrO4 nanorods
can affect the optical properties of PbCrO4 crystal.
4. Conclusions
In conclusion, we have successfully synthesized singlecrystalline PbCrO4 nanorods, with a rectangular cross section, a
typical length of 6–7 ␮m, a width of 80–150 nm, and a width-to-
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